Electromagnetic transmission systems often employ waveguide sections that are rigidly supported within a surrounding conduit. The waveguide requires the use of expansion joints to accommodate for temperature-induced expansion and contraction of the connected waveguide sections. For example, in long runs of low-loss circular waveguide operating in the TE.sub.01 mode, the variation in overall length of the waveguide with temperature may be on the order of inches. A slip joint or expansion joint is necessary to allow for this length variation in such a run where the ends are fixed. The waveguide may also be pressurized to form a barrier against the leakage of moisture into the waveguide. Thus the slip joint must be sealed to prevent loss of pressurization.
Various tapered joints have been employed in expansion joints for TE.sub.01 mode transmission waveguides. One such tapered expansion section is exemplified by U.S. Pat. No. 4,369,413 issued to Devan et al. The Devan expansion joint provides a mechanical expansion capability and taper transition between different operating waveguide diameters, while minimizing the generation of unwanted modes, especially the TE.sub.02 mode. Devan accomplishes this with a two-piece expansion joint; the first piece increases in diameter linearly from the diameter of the waveguide to which it is to be connected, to a diameter that is sufficiently small to prevent the generation of spurious modes (especially the TE.sub.02 mode) and equal to the inside diameter of the second piece. The second waveguide piece is concentrically and precisely fitted in slideable engagement with the first waveguide piece. The first and second waveguide pieces are arranged to permit sliding with respect to each other to provide the expansion and contraction capability. The second waveguide piece has at one end thereof an internal diameter that is equal to the enlarged diameter of the first waveguide piece. At the other end of the second waveguide piece there is a cosine or other specially shaped section to enlarge the joint to the diameter of the other waveguide to which it is to be connected.
Although this prior art expansion joint provides some expansion and contraction capability, it is limited by the frictional forces created by the precise fit where the outside surface of the first piece and the inside surface of the second piece are in contact. In the prior art designs this frictional force is necessary to provide alignment of the waveguide and the expansion joint and to prevent the emission of the propogating energy from the waveguide into free space. These disadvantages are overcome by the present invention.